This invention relates to optical links and is more especially, although not exclusively, concerned with high performance optical link configurations including wideband and/or high dynamic range optical links.
An optical link comprises an optical transmitter for transmitting a light signal along an optical path and an optical receiver for receiving the light signal. Such links can be used to transmit signals originating in parts of the electromagnetic spectrum having frequencies lower than that of the light signal. They may be used to convert microwave signals in the range from RF to millimeter wave into light signals and then transmit them.
Conventionally, wideband optical links are direct detection (DD) systems which use intensity modulated light sources. The performance of such (DD) links is usually limited by thermal noise in the optical receiver when low optical power levels are used and by optically generated noise (such as shot noise and relative intensity noise (RIN)) when high optical power levels are received.
In high performance optical links, whether digital or analogue, output wideband noise is largely caused by optically generated noise. An optical carrier from a light source is intensity modulated with the signal which it is to carry. In many cases the modulated optical power is much less than mean optical power from the light source, in which case there is a considerable level of standing optical power contributing to the noise and not to the signal.
Alternative methods of reducing the noise contribution from the unmodulated optical signal, such as bias offset or carrier filtering, result in a reduced modulation efficiency and an increased second order distortion making the optical link unsuitable for wideband systems.
In the known intensity modulated links, light from the source is incident on photodiodes in the optical receiver even if the optical transmitter is not receiving a modulating signal. In this event, a noise floor will always be present provided by optically generated noise. This restricts the dynamic range (DR) of the link.
The operation of high dynamic range, wideband optical links is defined with certain parameters including compression dynamic range (CDR) and spurious free dynamic range (SFDR). CDR is defined as the signal-to-noise ratio (SNR) at the 1 dB compression point For every 1 dB improvement in the noise level the CDR increases by 1 dB. SFDR is defined as the SNR at the driver power when the largest spurious tone equals the noise floor. This is generally specified for the two-tone third-order intermodulation signal. SFDR increases by 0.66 dB for every 1 dB improvement in the noise level.
Many potential applications for wideband optical links require a dynamic range of at least 10 dB better than can currently be realised. To achieve this performance it is necessary either to improve linearity of modulation which is applied to the light source or to reduce the noise floor.
To increase the signal noise ratio in an optical link for transmitting analogue signals it has been proposed in FR 2500972 to transmit the optical signal in the form of two optical signals: a first optical signal corresponding with at least a positive half wave of the signal to be transmitted and a second optical signal corresponding with at least a negative half-wave of the signal. Firstly the electrical signal is split into two electrical signals corresponding to the positive and negative half-waves and these are used to drive a respective LED to generate the two optical signals. The optical signals are then transmitted by a respective optical fibre to a pair of photodiodes which convert them back to electrical signals and combines them to form an electrical signal representative of the original signal. FR 2500972 states that the use of a symmetrical assembly of photodiodes in this way improves the signal to noise ratio since the signals add whilst the noise cancels.
The inventors have appreciated that whilst such an arrangement can offer an improved noise performance the bandwidth of the signal it is able to transmit is limited by the need to firstly convert the electrical signal into two electrical signals. The present invention arose in an endeavour to provide an optical link which at least in part solves this problem.
According to a first aspect of the invention an optical link comprises: an optical transmitter for generating intensity modulated optical radiation in dependance on an original electrical signal; an optical path for transmitting the modulated optical radiation; and demodulating means for converting the optical radiation into an output electrical signal representative of the original electrical signal; wherein the original electrical signal has an amplitude which varies from positive values to negative values and wherein the modulated optical radiation comprises a first varying optical signal representing a positive varying component of the original electrical signal and a second varying optical signal representing a negative varying component of the original electrical signal, the optical path carrying the first and second varying optical signals to the demodulating means which converts them into the output electrical signal; characterised by the optical transmitter comprising: a first optical modulator; a second optical modulator and a light source associated with the first and second optical modulators; the light source being operable to produce an unmodulated optical output and the first and second modulators being operable to intensity modulate the optical output in dependence on the original electrical signal to respectively produce the first and second varying optical signals; wherein each modulator has an optical transmission versus voltage characteristic having a substantially linear region and a threshold voltage above or below which substantially no light is transmitted by the modulator; and further comprising biasing means for biasing the first and second modulators to operate on a selected portion of said characteristic such that the first modulator is operable for positive varying components of the original signal and the second modulator is operable for negative varying components of the optical signal.
Generating each optical signal from the original signal using a separate modulator and associated light source in the way described eliminates the need to convert the original electrical signal and improves the bandwidth of the optical link. The bandwidth of the optical link of the present invention is determined by the operating speed of the modulator rather than electronic components which would otherwise be required to convert the electrical signal.
Preferably the optical path comprises an optical fibre. The first and second varying optical signals can be differentiated by having different wavelengths or different polarisation.
Thus in one embodiment the light source is operable to produce a first wavelength optical output which is applied to the first modulator and a second wavelength optical output which is applied to the second modulator such that the first and second varying optical signals have different wavelengths. Alternatively or in addition the optical fibre is polarising maintaining and the optical link further comprises means for placing one of the varying optical signals in a different polarisation state to that of the other before transmission along said fibre with such an arrangement the demodulating means includes a polarisation splitter for distinguishing between the first and second varying optical signals.
In one embodiment at least one of the modulators comprises an electro-absorption modulator. Preferably at least one the modulators comprises a series of cascaded electro-optic modulators to produce the desired modulator characteristic. It can be an interferometer.
Preferably the demodulator comprises first photoreceiver for converting the first varying optical signal into, the positive varying component and a second photoreceiver for converting the second varying optical signal into the negative varying component and means for combining the two components to produces the output electrical signal. Conveniently the second photoreceiver extracts the negative varying component in anti-phase to the first photoreceiver which extracts the positive varying component.
Preferably the original electrical signal is a microwave signal, radar signal or communication signal. It can be a television signal. The original electrical signal can originate from cable based or antenna based systems used in communication or radar systems.
According to a second aspect of the invention provides a method of transmitting an original electrical signal as an optical signal over an optical link in accordance the first aspect of the invention.